Insulating Rim Joists – Why and How

Rim joints, also known as band joists, are the wood that makes up the perimeter of the floor framing of your home. They are typically installed above the mudsill and provide homes with structural support around the floor frame. Every building needs rim joists as they close off open spaces of the floor and help make a home structurally sound.

Insulating your rim joists is one of the best things you can do to make your home energy efficient and save money on energy bills. In this article, we’ll go over why it’s important to insulate rim joists and whether or not you should consider taking on this home project yourself.

Why Insulate Rim Joists?

There are many reasons why people choose to insulate their rim joists. When you properly insulate your rim joists, you can increase your home’s energy efficiency. You will easily cut heat loss by keeping your home sealed and well-insulated. Not having your rim joists insulated properly could not only cause heat loss in the home, but can also lead to structural damage down the road. 

Methods

There are a few methods you can use to insulate your rim joists. The method you choose depends on your project budget, home type, age, and size, as well as your DIY skills. Here are the three main methods for insulating rim joists.

1. Interior Foam Insulation

Most builders will use interior foam insulation to insulate rim joists. Foam works to lower the risk of condensation while also reducing warm air from contacting a cold surface. There are several interior foams that are available to use including spray foam, XPS foam, EPS foam, and polyurethane foam. 

The type of interior foam you choose to use will depend on your project’s budget and the level of difficulty. Spray foam insulates rim joists and seals any air leaks at the same time. Closed-cell spray foam works well in colder climate areas. It reduces the risk of condensation but it will not keep the rim joist dry if a bulk moisture intrusion occurs.

2. Fiberglass

Although builders no longer use fiberglass, it was once a very popular insulation option. Fiberglass insulation is permeable which means that during the winter warm air can travel out of the home and come into contact with the cool air, creating condensation. Fiberglass provides thermal resistance however, it isn’t able to prevent condensation. 

It’s generally recommended not to use fiberglass for new projects due to the downsides of the material. It’s included here for completeness and because you may encounter its use in older homes. 

3. Rigid Foam

Rigid foam is the cheapest foam insulation option, however, it does come with its drawbacks. It is the most difficult to manipulate and can be tricky to install in tight and awkward areas. The three types of rigid foam include expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate. 

EPS is a great budget-friendly option if you’re looking to save on costs. If you’re in search of a sustainable, environmentally-friendly option, polyisocyanurate is the best option while XPS is the least eco-conscious choice. 

Considerations

There are a few essential considerations that you should be aware of before taking on this home project. Being knowledgeable about all of the safety codes, requirements, and other important factors will help make insulating your rim joists a seamless and problem-free process.

As with any home improvement project you should check your local laws and regulations before starting.

Fire Safety

When insulating rim joists, it’s vital to be aware of and to meet all of the safety requirements. There are some types of insulation that are extremely flammable which is why you need to be up to date on the latest fire codes and regulations. 

For rigid foam, builders are required to cover it with a thermal barrier like a layer of ½ in drywall. With spray foam, there is no thermal barrier required as long as the spray is no thicker than 3 ¼ inches. Mineral wool works as a great insulation and thermal barrier.

Venting

You can drill a hole through your rim joists if you need to run wires, a vent, or pipes to the outside of your home. Make sure that after you drill the hole and complete the installation, you insulate around the hole and seal it completely.

Spray Foam

When it comes to spray foam, you need to be careful and use safety equipment. You will need to wear goggles, gloves, a respirator, and clothes that cover all of your skin. Spray foam is very difficult to remove, so make sure you cover any exposed items in your home.

Condensation

During the winter, rim joists become cold and if they come into contact with warm interior water vapor, condensation occurs. Since rim joists are very vulnerable to condensation, it’s essential that they are properly insulated. When you properly insulate your rim joists, you create a barrier so that the warm interior water vapor doesn’t come into contact with the joists. Too much condensation can create mold, which is harmful to your home and your family’s health.

Rot

Rot is much more common in older houses that don’t have a break between the mudsill and foundation. When there is no break in between these two areas, the rim joists are at a much higher risk of developing dampness and will start to rot. 

DIY vs Hiring?

Whether you decide to insulate your home’s rim joists yourself or prefer to hire a professional is entirely up to you and how comfortable you are taking on the task yourself. Are you comfortable with your construction skills? If you are, you could DIY your rim joists. If you’re not entirely comfortable with the process of insulating rim joists yourself, it could be best to hire a professional.

It’s important to consider your house’s size, age, and build before making the decision. For a two-story house, you’ll have two sets of rim joists to insulate instead of just one. In this case, it may be best to hire a cellulose-insulation contractor.

Properly insulating your rim joists can help save you money on your energy bills, seal in your home’s heat, and prevent condensation that can lead to rot and mold. Both rot and mold can lead to structural damage as well as negative health effects which you’ll want to avoid at all costs. If you’re comfortable with your handyman skills, you can choose to insulate your rim joists on your own but if you’re not sold on taking on the project yourself, reach out to a professional.